System and method for collimating and redirecting beams in a fiber optic system

ABSTRACT

A connector to an optical fiber comprises a prism, a ferrule and an aspheric lens. The prism includes a triangular wedge element having a first surface, a second surface and a base. The ferrule guides the optical fiber so as to contact the optical fiber with the first surface of the prism. The aspheric lens is integrated on the second surface, the integrated aspheric lens being positioned so that the prism serves to redirect a light beam at an angle relative to an axis of the optical source input through total internal reflection by utilizing the base of the triangle wedge element. The aspheric lens serves to collimate the redirected light beam or focus the light beam before being redirected. This arrangement may, for example, be used within a WDM system to multiplex and de-multiplex several wavelengths of light, using a “zig-zag” optical path configuration and thin film filters to separate the wavelengths.

This application claims the benefit of U.S. Provisional Application No.60/244,941, filed Nov. 1, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical communications, and moreparticularly, to a system and method for collimating and redirectingbeams in a fiber optic system.

2. Discussion of the Related Art

The integration of aspheric lenses within connectors designed to couplelight beams from one optical fiber to another, and manufacturable usinginjection molding of optically transparent plastic, is well known. Theseapplications generally address the need to maintain alignment betweenthe axis of an optical fiber and a light beam output therefrom. Onegroup of previous known designs describes the placement of a concentricplano-convex lens with the flat side of the lens near or against a fibercore. Another group of known designs is shown in FIG. 1a. This typeutilized the placement of plano-convex lens with the flat side away froma fiber core and an air gap cavity separating the lens and the fiber.Shown in FIG. 1a is a cross-sectional view of a prior opticalfiber-connector assembly, comprising the integration of an optical fiber2 having a fiber core 3 with a fiber connector housing 1. The opticalfiber 2 and a collimating lens 7 are positioned so that a cavity 5 isformed between them, with the flat side of the collimating lens 7 beingaway from the fiber core 3. As an extension to the design shown in FIG.1a, the design in FIG. 1b illustrates the use of a wedge 6 to redirectthe beam at 45° using total internal reflection (TIR) after the beam iscollimated by the collimating lens 7.

Similar assemblies designed to couple light directly from a verticalcavity surface emitting laserdiodes (VCSEL) into a multimode fiberand/or couple light from an optical fiber directly onto a photodetectoralso appear in the prior art. One such assemblies has a design with aconcentric TO can ferrule, lens, and fiber ferrule elements. Designs ofthis nature, applying specifically to coupling between VCSELs andphotodiodes mounted inside TO cans and optical fibers, are devised foruse in serial data links rather than wavelength division multiplexing(WDM) systems.

More recent designs involving wavelength division multiplexers (WDM)employing thin film filter (TFF) channel separation and a “zig-zag”configuration may be subdivided into two types of designs. The firsttype of designs centers around the use of optical waveguides, consistingof regions of high index material (core) surrounded by a lower indexmaterial (cladding), to route the light along the “zig-zag” waveguides.The second type of designs involves those designs that depend oncollimation and free-space “zig-zag” optical routing. In implementation,the collimation, redirection, and focusing of light relevant to thesecond type of designs, or the free-space “zig-zag”multiplexer/de-multiplexer designs, differ drastically from the firsttype of designs, or the waveguide-based solutions. Prior art involvingTFF-based wavelength division multiplexers (WDM) that employ afree-space “zig-zag” configuration generally applies to fiber-to-fiberapplications such as optical switches, branch filters, and add-dropmultiplexers. Most of these designs have a planar topology that is notwell suited for current injection molding technology. Therefore, thereis a need for a system and method that utilize free-space “zig-zag”optical routing while being suited for current injection moldingtechnology.

There is little, if any, prior art that describes a design for aTFF-based optical WDM transceiver that uses injection molding oftransparent plastic to construct an integrated optical assembly. Oneexample of a related design was presented by B. Wiedemann at the IEEE802.3ae Interim Meeting in 2000. The input collimator of this design isconsistent with the air-gap cavity design mentioned earlier with respectto FIGS. 1a and 1 b. A serious disadvantage of this design and thedesigns of FIGS. 1a and 1 b is the absence of a ferrule to guide thefiber along the axis of a collimating lens. A small shift in fiberposition results in a serious misalignment of the collimated beams. Ifstandard injection molding techniques were used to manufacture thedesign, addition of the ferrule that is necessary to refine the designwould be extremely difficult because shaping the lens and ferrule on thesame “slide” would generate an undercut condition.

An additional problem is depicted in FIG. 2, which shows that acollimating element of related design consists of a lens surfacepositioned on a tilted base of refractive material. The diagram suggeststhat, by design, the chief optical ray of a beam 8 from a point source 4strikes the surface of a lens 7 near its center and refracts into therefractive material 9 at an angle equal to the tilt angle of the base onwhich the lens surface is mounted. The tilt angle of the base is used toredirect the chief ray of the beam 8 to the desired angle, while thecurvature of the lens 7 is used to collimate the beam 8. Because thechief ray is deliberately designed to penetrate the surface of the lens7 off its axis of symmetry, the quality of beam collimation issacrificed. It is impossible to eliminate aberration in the beam 8 evenif aspheric terms are added to the sag equation defining the lens 7.Aberration is especially great for sources of large numerical aperture,for large tilt angles, and for sources displaced slightly from theoptimal position. Therefore, there is a need for a system and methodthat collimates and redirects beams in a fiber optic system in a moreefficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the present invention and,together with the description, serve to explain the principles of thepresent invention:

FIG. 1a illustrates a cross-sectional view of a prior opticalfiber-connector assembly;

FIG. 1b illustrates a cross-sectional view of another prior opticalfiber-connector assembly;

FIG. 2 illustrates a prior design intended to collimate light from apoint source, such as an optical fiber;

FIG. 3a shows an optical subassembly for redirecting and collimatingoutput from an optical fiber according to an embodiment of the presentinvention;

FIG. 3b shows an optical subassembly for redirecting and coupling acollimated beam into an optical fiber according to an embodiment of thepresent invention;

FIG. 4a shows an optical subassembly for collimating and redirecting anoutput beam from a surface emitting laser according to an embodiment ofthe present invention;

FIG. 4b shows an optical subassembly for redirecting and coupling acollimated beam into a photodetector according to an embodiment of thepresent invention;

FIGS. 5a and 5 b illustrate schematically designs for a four-channelwavelength division multiplexer/de-multiplexer with a fiber opticinput/output according to an embodiment of the present invention;

FIG. 6 shows a case where the magnification of an optical system is notequal to one according to an embodiment of the present invention;

FIGS. 7a and 7 b show a four channel CWDM transceiver integrated with aconnector housing suitable to receive a fiber optic connector accordingto an embodiment of the present invention;

FIG. 8 shows a complete transceiver module in which embodiments of thepresent invention may function; and

FIG. 9 illustrates an optical subassembly for redirecting andcollimating output from an edge-emitting laser according to anembodiment of the present invention;

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the present invention will be described in conjunctionwith the preferred embodiments, it will be understood that they are notintended to limit the invention to these embodiments. On the contrary,the present invention is intended to cover alternatives, modificationsand equivalents, which may be included within the spirit and scope ofthe invention as defined by the appended claims. Moreover, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, the invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, and components have not been described in detail asnot to unnecessarily obscure aspects of the present invention.

Embodiments of the present invention are directed to addressing theaforementioned drawbacks associated with collimating and redirectingbeams in a fiber optic system. An embodiment of the present invention isdirected to a fiber collimator for redirecting and collimating a lightbeam from an optical source input. The fiber collimator comprises aprism with a triangular wedge element having a first surface, a secondsurface and a base, a ferrule to guide the optical source input so as tocontact the optical source input with the first surface of the prism,and an aspheric lens integrated on the second surface. The integratedaspheric lens is positioned so that a chief ray of the light beam passesdirectly through the axis of the aspheric lens. The prism serves toredirect the beam at an angle relative to an axis of the optical sourceinput, with the base of the triangle wedge element redirecting the lightbeam by total internal reflection (TIR). The aspheric lens then servesto collimate the redirected light beam.

In one embodiment, the present invention is further directed to a fibercoupler for redirecting and coupling a light beam into an optical fibercore of an optical fiber. The fiber coupler comprises a prism, anaspheric lens and a ferrule similar to those in the fiber collimator.The aspheric lens receives a light beam, and is positioned so that thelight beam is focused after passing through the aspheric lens, creatinga focal spot image. The base of the triangle wedge element of the prismserves to redirect the focused light beam by total internal reflection(TIR) at an angle relative to an axis of the optical fiber, the focusedlight beam becoming coupled into the optical fiber core. The ferruleguides the optical fiber so as to contact the optical fiber core withthe first surface of the prism at or near the location of the focal spotimage.

Embodiments of the invention are also directed to a collimating opticalsubassembly and a focusing optical subassembly. Both are fabricated ofoptically transparent material and integrated as a single part usinginjection-molding techniques. The former is for collimating andredirecting a divergent light beam from a point source; it comprises anaspheric lens, a spacer element and a wedge element. The aspheric lensreceives and collimates a divergent light beam, creating a collimatedlight beam. The spacer element is positioned above the aspheric lens,and the wedge element is positioned above the spacer element. The wedgeelement refracts the collimated light beam into air at an angle relativeto the axis of the aspheric lens consistent with Snell's law. On theother hand, the focusing optical subassembly is for redirecting andfocusing a collimated light beam. The focusing optical subassemblycomprises a wedge element, a spacer element and an aspheric lens. Thewedge element receives a collimated light beam from outside. The spacerelement is positioned below the wedge element, and the aspheric lens ispositioned below the spacer element. The collimated light beam receivedby the wedge element travels in air at an angle relative to an axis ofthe aspheric lens. The wedge element redirects a chief ray of thecollimated beam through the spacer element along the axis of theaspheric lens. The aspheric lens focuses the collimated light beam to apoint along its axis.

Another embodiment of the present invention is directed to a transceivercomprising alignment ferrules for optical fibers, a fiber connectorhousing, and a ledge to precisely control the planar orientation of aprinted circuit board. Using a combination of aspheric lenses andprisms, and in particular, the fiber coupler, the fiber collimator, thecollimating optical subassembly and the focusing optical subassembly,the transceiver is able to collimate and route light from two or moreVCSELs or edge-emitting lasers through a “zig-zag” configuration as wellas to redirect and focus the combined light onto the core of a singleoptical fiber. Moreover, collimating and routing light from a singleoptical fiber, having multiple wavelength components, through afree-space “zig-zag” configuration and redirecting and focusing thelight at each individual wavelength onto a different photodetector areachieved.

The WDM transceiver module described herein is paraxial by design, usingaspheric lenses to compensate for spherical aberration, and opticallyoptimized for alignment tolerances of the laser sources andphotodetectors. This is done by adjusting the optical focal lengths ofthe lenses to magnify or de-magnify the image of the laser source(transmitter) or fiber core (receiver) on the respective image plane.Appropriate magnification may be used to increase the tolerance of theoptical system to misalignment of the laser sources and/orphotodetectors, and/or to match the numerical aperture of the lasersource to that of the optical fiber into which the light is coupled.This optimization maximizes the overall alignment tolerance of theoptical subassembly with the substrate on which the lasers andphotodetectors are attached, and/or the coupling efficiency of thelasers to the optical fiber. This optimization technique, the specificlayout of fiber ferrules, prisms and lenses integrated as one part withthe ledge to aid in positioning on a printed circuit board are enablingfeatures for mass production of low-cost WDM transceivers using currentstate-of-the-art optical-quality injection molding techniques.

According to a further embodiment of the present invention, a moldassembly for fabricating an integrated optical assembly as a singleinjection-molded part is provided. The mold assembly comprises first andsecond mold halves and a single slider. The first and second mold halvesare arranged to mate with each other, forming a draw direction orientedparallel to axes of aspheric lenses of a focusing optical subassembly ofan optical de-multiplexer and a collimating optical subassembly of anoptical multiplexer. The single slider is used to form ferrules for afiber collimator of the optical de-multiplexer and a fiber coupler ofthe optical multiplexer as well as to form the connector housing. Thefirst mold half is used to shape wedges of the collimating and focusingoptical subassemblies and to shape aspheric lenses of the fibercollimator and the fiber coupler. The second mold half is used to shapetotal internal reflection surfaces of the fiber collimator and the fibercoupler and to shape the aspheric lenses of the collimating and focusingsubassemblies.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention means that a particular feature, structure orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” appearing in variousplaces throughout the specification are not necessarily all referring tothe same embodiment.

Embodiments of the present invention are directed to systems and methodsfor collimating and redirecting beams in an optical system. In general,the present invention is applicable whenever light from a pointsource—e.g., laser, optical fiber—needs to be collimated, directedoff-axis, and refocused to form an image of the source with good qualityand carefully controlled position, size and numerical aperture. Thepresent invention is also applicable when the alignment tolerance of thesource and the image relative to the optical system needs to beoptimized. For example, embodiments of the present invention areutilized in a wavelength division multiplexing (WDM) system to multiplexand de-multiplex several wavelengths/channels of light, wherein WDMtransceiver modules are optically optimized for the alignment toleranceof the laser sources and photodetectors. The optimization maximizes theoverall alignment tolerance of the optical subassembly with thesubstrate on which the lasers and photodetectors are attached. Thistechnique, along with the specific layout of prisms and lenses, areenabling features for mass production of optical subassemblies andfree-space coupled WDM transceiver modules.

With reference now to FIG. 3a, an optical subassembly 100 a according toan embodiment of the present invention is provided. In the embodiment,the optical subassembly 100 a is a fiber collimator that is designed toredirect and collimate an output of a divergent light beam 20 from anoptical fiber 10, creating a collimated output 25. The opticalsubassembly 100 a comprises a prism 30, an aspheric lens 40 and analignment ferrule 50. The optical fiber 10 directs the divergent lightbeam 20 into the optical subassembly. Prior to collimation, the prism30, having an index of refraction n, is used to redirect the divergentlight beam 20 at a specific angle (φ) relative to the axis normal to theincoming optical axis using total internal reflection (TIR). TIR resultsas divergent light beam 20 hits a TIR surface 80 of the prism 30. In oneimplementation, the angle φ must satisfy the following condition:

2 arcsin(1/n)−π/2<φ<π/2

The prism 30 may, for example, be made of polycarbonate because of itshighly transparent characteristic, with the index of refraction n beingapproximately 1.57. If this case, the angle φ must be greater than−10.87 degrees.

As shown in FIG. 3a, the prism 30 has a spacer element 60, having alength D. The length D of the spacer is defined in such a way that thefolded optical path of light passing through the aspheric lens 40 placesthe focus of the lens 40 at or near the input wall of the prism 30. Thespacer element 60 is an important feature to the design of the opticalsubassembly 100 a, enabling convenient tuning of the focal length andnumerical aperture in this portion of an optical system whilemaintaining a well-defined focal plane, which is important for alignmentpurposes. However, it should be apparent to one in the art thatdepending on the specific arrangement of the optical fiber 10 and thestructures of optical subassemblies in accordance with other embodimentsof the present invention, the spacer element 60 may not be needed.

According to an embodiment of the present invention, the alignmentferrule 50 is attached to the prism 30 to ensure that the optical fiber10 is optimally aligned to the prism 30. As shown in FIG. 3a, thisalignment is defined by having the chief ray from the fiber propagatedirectly along the axis of symmetry of the lens 40. In one embodiment,this is ensured by the geometry of the prism 30, designed such that anisosceles triangle (“abc”) wedge is provided. Here, the isoscelestriangle abc is symmetric about the vertex “c”, i.e., the length of sideca is equal to the length of side cb. The isosceles triangle abc wedgeis provided to ensure that the chief ray enters and exits the opticalsubassembly 100 a at a normal incidence. This preserves the independenceof the angles of the isosceles triangle abc on the choice of refractivematerial used to construct the optical subassembly. Used in conjunctionwith the spacer element 60, the isosceles triangle abc wedge can be keptat a constant dimension, while the spacer element 60 provides a meansfor adjusting the optical path length from the optical fiber 10 to theaspheric lens 40. This allows the focal length of the lens 40, andthereby the radius of the collimated beam 25, to be adjusted whilekeeping the dimensions of the isosceles triangle abc wedge constant. Inother embodiments, other types of triangular shaped wedge may be used.

In one implementation, the optical subassembly 100 a, consisting of thealignment ferrule 50, the prism 30 and the aspheric lens 40, ismanufactured as one part, thereby minimizing alignment inaccuracy. Theprism 30, the aspheric lens 40 and the alignment ferrule 50 may be madeby a standard injection molding process using, for example,polycarbonate, polyolefin, or polyethylimide. On one hand, polycarbonateis often used because of its high transparency. On the other hand,polyethylimide is often used because of its high temperature operatingcharacteristic and low coefficient of thermal expansion (CTE).

The optical subassembly described above can naturally be operated inreverse to redirect and focus a collimated beam into an optical fiber.FIG. 3b illustrates an optical subassembly 100 b for redirecting andcoupling collimated beam into an optical fiber according to anembodiment of the present invention. In the embodiment, the opticalsubassembly 100 b is a fiber coupler created for redirecting andfocusing a collimated beam 25′ whose chief ray enters along the axis ofsymmetry of an aspheric lens 40. The focal length of the lens 40 isdesigned to produce an image spot with a numerical aperture matching thenumerical aperture of an optical fiber 10. The prism 30 is utilized toredirect a convergent light beam 20′, which is formed from thecollimated beam 25′ passing through the aspheric lens 40 and redirectedby a TIR surface 80. An alignment ferrule 50 and a spacer element 60 areused to precisely position the optical fiber 10 at the focus of theoptical subassembly 100 b.

FIG. 4a illustrates an optical subassembly 200 a for collimating andredirecting an output beam from a surface emitting laser according to anembodiment of the present invention. The optical subassembly 200 acomprises a prism 230, an aspheric lens 240, and a spacer 260 positionedin between the prism 230 and the aspheric lens 240. In one embodiment,the optical subassembly 200 a is used for collimating and redirecting anoutput beam from a vertical cavity surface-emitting laser (VCSEL) 250.Examples of the laser sources of interest for this application includes,but are not limited to, VCSELs emitting in the 850 nm band, VCSELsemitting in the 980 nm band, and VCSELs currently in development at the1300 nm band. In contrast to the prior art design shown in FIG. 2, anon-tilt aspheric lens 240 is used to collimate the output beam from theVCSEL 250, while a flat, tilted surface 235 of the prism 230 is used toredirect the collimated beam to a desired angle. This effectivelyuncouples the two separate tasks of redirection and collimating in thepresent design, making the design “paraxial,” i.e., the beam falls onthe refracting surface close to and almost parallel to the axis. Theabsence of tilt in the aspheric lens 240 and the addition of a second,flat surface 235 to perform the function of redirecting the beam are keydifferences between the present invention and prior designs. Thissignificantly improves the collimated beam quality, enabling the outputbeam to remain well collimated even for a source having a largedivergence angle and/or significant offsets in the position of thesource. The result is a greatly improved tolerance to misalignment.

Like the optical subassembly 100 a described in FIG. 3a, the opticalsubassembly 200 a shown in FIG. 4a, can be used in reverse to redirectand focus a collimated beam. FIG. 4b illustrates an example of such anoptical subassembly 200 b. A collimated beam is incident to a flat,tilted surface 235 of the prism 230 at a specific angle. The opticalsubassembly 200 b, being paraxial in nature, makes use of the flat,tilted surface 235 to redirect the collimated beam and an aspheric lens240 to focus the beam. This results in a diffraction-limited image beingfocused onto a photodetector 210. Since photodetectors utilized forhigh-speed optical modulation typically have small active area, effortsto minimize the size of the image are important to maximize both theoptical energy detected and the tolerance to detector misalignment. Withthe paraxial optical subassembly 200 b, the size of the image isminimized at the photodetector 210. Typically, a spacer 260 is providedbetween the aspheric lens 240 and the prism 230. One of the functions ofthe spacer 260 is to allow molten optically transparent material toeasily flow through a mold for fabricating the optical subassembly 200 aand/or the optical subassembly 200 b during injection moldingmanufacturing processes.

FIGS. 5a and FIG. 5b illustrate schematically designs for a four-channelwavelength division multiplexer/de-multiplexer with a fiber opticinput/out according to embodiments of the present invention. Thefour-channel wavelength division multiplexer/de-multiplexer uses thinfilm filters (TFFs) in a “zig-zag” scheme to perform channel separation.FIG. 5a depicts an optical demultiplexer, or a receiver, that consistsan optical subassembly 100 a as depicted in FIG. 3a, a glass plate 300,a set of TFFs 270 a-270 d and a set of focusing optical subassemblies200 b as depicted FIG. 4b. With the prism 30 and aspheric lens 40, theoptical subassembly 100 a, or the fiber collimator, redirects andcollimates a divergent beam from an optical fiber 10 at a specific anglerelative to normal to the input axis of the optical fiber 10. In theembodiment, the light beam contains multiple wavelengths, and fourfocusing optical subassemblies 200 b and four TTFs 270 a-270 d areprovided. From the optical subassembly 100 a, the collimated,multi-chromatic beam refracts into the glass plate 300, which has a highrefractive (HR) coated surface 310 for reflecting the beam. The HRcoated surface 310 is coated with a broadband high reflective (HR)coating, and it is coated on the side opposite to the side from with thebeam enters. The bandwidth of the HR coating includes all thewavelengths to be de-multiplexed. The input angle of the collimated beamis controlled to allow the collimated beam to reflect back-and-forthwithin the glass plate 300 with a specific spacing d. This is determinedby the angle of a TIR surface of the prism 30, the refractive index ofthe glass plate 300, the refractive index of the TFF substrates 270a-270 d, and the thickness of both the glass plate 300 and the TFFsubstrates 270 a-270 d.

In the embodiment, each of four TFFs 270 a-270 d is highly reflectiveover the same bandwidth as the above-described HR coating surface 310 onthe glass plate 300, except within a narrow passband centered at one ofthe four wavelengths that comprise the input beam. The passband of theTFFs 270 a-270 d should be wide enough to allow for laser wavelengthdrift with temperature, manufacturing error, etc. The center wavelengthsshould be spaced sufficiently far apart, so that negligible overlapexists among the passbands of the TFFs 270 a-270 d. The width of each ofthe TFFs 270 a-270 d used in the embodiment of the present inventionshown in FIGS. 5a and 5 b may, for example, be 10-15 nm and the spacingmay, for example, be 20-25 nm. Further, the design of the multi-layerdielectric structure, used to construct passbands of the TFFs 270 a-270d employed in the embodiment, is optimized for the specific angle ofincidence expected for the “zig-zag” scheme. This optimization minimizesany variation in reflection and transmission due to the polarizationstate of the incident light.

In one embodiment, the TFFs 270 a-270 d are course wavelength divisionmultiplexing (CWDM) filters, or so-called wide wavelength divisionmultiplexing (WWDM) filters, that are used in a CWDM system. In otherembodiments, the TFFs 270 a-270 d are dense wavelength divisionmultiplexing (DWDM) filters, that are used in a DWDM system, where lightat different wavelengths is closely packed. In one implementation, thesubstrate of each TFFs 270 a-270 d is composed of the same refractivematerial as the glass plate 300. In other implementations, the TFFs 270a-270 d and the glass plate 300 have different refractive indexes. InFIGS. 5a and 5 b, each of the TFFs 270 a-270 d is positioned such thatits bandpass coating faces away from the glass plate 300. However, thebandpass coating may be placed in contact with the glass plate 300 inother embodiments. In one implementation, an index matching epoxy may beused to attach the set of TFF substrates 270 a-270 d to the glass plate300.

In operation, a wavelength component of the collimated beam is shown topropagate back-and-forth within the glass plate 300 of the opticaldemultiplexer of FIG. 5a. The propagation ends when the collimated beamis incident onto a particular TFF with a bandpass coating that allowsthe particular wavelength component to pass. As illustrated in FIG. 5a,the TTF 270 a is the particular TTF for a particular wavelengthcomponent. After passing through TFF 270 a, the wavelength componentpasses through a focusing optical subassembly and is focused onto aphotodetector 210. The photodetector 210 may, for example, be aphotodiode. Although not shown for sack of clarity, each of the otherwavelength components (not shown) in the input beam passes through adifferent TFF and is directed and focused onto a different photodetector(not shown). Preferably, a photodetector is positioned beneath eachaspheric lens 240 to detect the wavelength component passingtherethrough.

FIG. 5b depicts a four-channel wavelength optical multiplexer accordingto an embodiment of the present invention. Light from any one of VCSELs250 is first collimated, then redirected into a “zig-zag” optical path,and finally coupled into an optical fiber 10. The VCSELs 250 are mountedon a line below the multiplexer. Use of several VCSELs and TFFs havingnon-overlapping passbands, each centered at the emitting wavelength ofits corresponding VCSEL, enables the design of a wavelength divisionoptical multiplexer (WDM). One VCSEL is positioned beneath each asphericlens to introduce its light beam into the WDM system. For sack ofclarity, only one light beam and one VCSEL are shown. The light beamemitted from a VCSEL 250 passes through its corresponding opticalassemblies 200 a, like the one described in FIG. 4a. The light iscollimated by the aspheric lens 240 and redirected by the prism 230 intothe glass plate 300. Upon entering the glass plate 300, the light beampasses through the TFF 270 a. Light beams from other VCSELs enter theglass plate 300 in a similar fashion. Since the TFFs 270 a-270 d havenon-overlapping passbands, different wavelength components are extractedfrom the light beams. Inside the glass plate 300, light with differentwavelength components travels in a “zig-zag” optical path while beingreflected by the HR coating 310 and the TFFs 270 a-270 d. After leavingthe glass plate 300, the light with different wavelength components arecoupled into an optical fiber 10 by an optical subassembly 100 b, likethe one described in FIG. 3b.

Of importance to the transceiver design depicted in FIG. 5a and FIG. 5bis the alignment of the VCSELs 250 and photodetector apertures 210 totheir corresponding aspheric lenses 240. Misalignment often resultsbecause of the manufacturing tolerances of optical subassemblies and/orglass plates, thermal expansion, placement accuracy of a die bonder usedto position the VCSEL 250 and photodetector 240, etc. Efforts tomaximize the amount of light coupled into the optical fiber 10 despiteany misalignment of the VCSEL 250, and/or to maximize the amount oflight incident onto the photodetector 210 despite any misalignment ofthe photodetector 210, are necessary to make the transceiver design morereadily manufactured. In the present invention, several design featuresare specifically utilized for this purpose. For example, to ensure anoptimally collimated beam, despite small variation in the position ofthe VCSEL 250 in FIG. 4a, the aperture of the aspheric lens 240 is madelarger than the waist of the output beam, that is

R>ƒ ₂ tan(sin⁻¹(NA/n)),

where R is the aperture of the aspheric lens 240 and ƒ₂ is the focallength of the aspheric lens 240. The lens parameters for the asphericlens 40 in FIG. 3b are optimized by assuming a source with a numericalaperture (NA) that completely fills the full aperture of the lens, givenby the following expression:

NA=n sin(tan⁻¹(R/ƒ ₁)).

This ensures that the image of the source remains undistorted even ifthe source is displaced from its optimal position. When the VCSEL 250 isdisplaced, the undistorted, diffraction-limited image, whose size isproportional to the size of the source, simply translates in the imageplane an amount proportional to the displacement of the source. Theconstant of proportionality for both size and displacement is themagnification of the entire optical system, given by the ratio of thefocal length ƒ₁ of the optical subassembly 100 b and the focal length ƒ₂of the optical subassembly 200 a. Similar arrangements and analysisapply to optical subassemblies depicted in FIGS. 4b and 3 a. If thefocal length of the aspheric lens 40 and the aspheric lens 240 areequal, for example, the laser source from the VCSEL 250 is imaged atactual size onto the fiber core of the transmitter fiber 10, and thefiber core of the receiver is imaged at actual size onto thephotodetector 210. The image in each case moves one micron for everymicron of source displacement.

FIG. 6 shows a general case, where the magnification of the opticalsystem is not equal to one, i.e., the laser source or the fiber core isnot imaged at actual size. If the VCSEL 250, or the laser source, isdisplaced from its ideal position by a distance X_(source), then theimage 510 of the VCSEL aperture on the fiber core 500 moves a distanceX_(source) ƒ₁/ƒ₂. Approximately half the power (i.e. 3 dB) is lost inthe transmitter when X_(source) ƒ₁/ƒ₂=D_(fiber)/2, where D_(fiber) isthe diameter of the optical fiber. The alignment tolerance for placingthe laser source is, therefore, X_(source 3 dB≈)D_(fiber)ƒ₂/2ƒ₁. Itwould be desirable to minimize the magnification in order to maximizethe alignment tolerance; however, the choice of magnification alsoimpacts the numerical aperture (NA) of the source image projected ontothe fiber core. It is undesirable to project an image to the fiber corewith a NA larger than that of the fiber because light will be lost. TheNA, NA_(source), of the source image and the NA, NA_(fiber), of theimage projected onto the fiber core are related through the followingexpression:

NA _(fiber) =NA _(source)ƒ₂/ƒ₁

Because of this, the minimum desirable magnification is given by

ƒ₁/ƒ₂ =NA _(source) /NA _(fiber.)

The alignment tolerance of the source in this case becomesX_(source 3dB≈)D_(fiber)NA_(fiber)/2NA_(source.) In one implementationof the present invention, this constraint is used when designing theprescription for the lens 40 in the optical subassembly 100 b used tocouple collimated light into the optical fiber 10 and the lens 240 inthe optical subassembly 200 a used to collimate the light from the VCSEL250. For example, a typical 50 micron diameter multimode fiber having aNA near 0.2, and a typical VCSEL, having a NA near 0.26, suggests amagnification near 1.3. This translates into an alignment tolerance of±20 microns for the VCSEL 250.

FIGS. 7a and 7 b show perspective views of a four-wavelength CWDMtransceiver integrated with a housing suitable for receiving a duplexfiber optic connector according to an embodiment of the presentinvention. FIG. 7a is a cross-sectional view of the four-wavelength CWDMtransceiver with the housing, and FIG. 7b is a perspective view of thefour-wavelength CWDM transceiver with the housing looking from thebottom of the transceiver. The assembly/device shown in FIGS. 7a and 7 bconsists of at least one of the optical subassemblies 200 a, 200 bdescribed in FIG. 5a and FIG. 5b, respectively, integrated with aconnector housing 400 suitable to receive a duplex fiber connector orany other fiber optic connector. Shown in FIG. 7a are slots used foraccurately placing TTFs 270 over the optical subassemblies 200 a or 200b. These slots are created at a distance equal to or substantially equalto the TFF substrate thickness. The square cavity directly above theprism/lens components, and above the TFFs 270, is where the HR coatedglass plate 300 is positioned. In a preferred implementation, theassembly depicted in FIG. 7a and FIG. 7b is manufactured as a singlepart, using plastic (e.g. polycarbonate or polyethylimide)injection-molding techniques. In these techniques, plates containingcavities where plastic will ultimately reside are brought together alonga “direction of draw,” which is designated by reference numeral 600.This forms a closed mold. One or more “sliders,” designated by referencenumeral 610, manufactured with surface shapes relevant to the plasticpart of interest, may be inserted into the closed mold along axes otherthan that of the draw direction 600. Mold inserts for optical surfacesmay be manufactured separately using high precision, diamond-tipmachining techniques and inset into the rest of the mold. Once thecomplete volume is defined, molten plastic is injected through one ormore gates, with the pressure of the plastic displacing the air in thecavity out through one or more vents. Once the plastic part cools, thesliders 610 retract and the plates separate. The part is then ejectedand the cycle repeats.

While the design rules for manufacturing molds suitable for plasticinjection molding are well known, special care is required to design asingle-piece WDM transceiver mold with the optical functionalitydescribed in FIG. 5a and/or FIG. 5b. Shown in FIGS. 7a and 7 b, the lensand prism surfaces in the preferred implementation of the presentinvention are easily cast using only the two mold faces that meet alongthe direction of draw 600. A single “slider” surface is used to cast theinside of the connector housing, including the alignment ferrule 50 usedto guide the optical fiber. Diamond-turned inserts are, for example,used to define all optical quality surfaces including the lenses 40,prisms 30, and TIR surfaces 80. In addition, a plastic spacer 260 is,illustratively, placed between the redirecting prisms 230 and theaspheric lenses 240. The spacer 260 is included to provide, as one ofits functions, an adequate flow channel for the molten plastic.

FIG. 8 shows a complete transceiver module according to an embodiment ofthe present invention. The complete transceiver module includes a dualfiber optic connector 710, an injection-molded optical assembly 720, aprinted circuit board (PCB) 730, and a metal shield for minimizingelectromagnetic interference (not shown). Optical fibers 10 a, 10 b areconnected to the dual fiber connector 710. One optical fiber in the dualconnector 710 is for the receiver, and the other one is for thetransmitter. As described, the optical assembly 720 is preferably aone-piece injection-molded optical subassembly with a connector housing400. The dual fiber connector 710 slides into the connector housing 400.The PCB 730 is aligned with the one-piece injection-molded opticalsubassembly in the optical assembly 720. On the PCB 730, there are lasersources, photodetectors, chips for processing electrical signals, othercircuitry, etc. To aid the alignment, a ledge structure is provided in aplane parallel to the plane tangential to, and passing through, the apexof the aspheric lenses of the collimating and optical subassemblies inthe optical assembly 720. The ledge structure allows the PCB 730 to beinserted and to be parallel to the aspheric lenses within a few micronsof tolerance.

While the preferred implementation of the present invention utilizes aVCSEL as the optical source for each wavelength in the WDM transmitter,an edge-emitting laser (EEL) may alternatively be used. FIG. 9illustrates an optical subassembly, and more particularly, a collimatingelement useful for redirecting and collimating output from an EEL 100according to an embodiment of the present invention. Similar to theoptical subassembly 100 a depicted in FIG. 3a, the collimating elementcomprises an aspheric lens 140 and a prism 130 with a spacer 160. Thedivergent light beam 120 from the EEL 100 is redirected at a specificangle, φ, relative to normal to the optical axis of the EEL 100 by TIRusing a prism 130. The divergent beam 120 is then collimated using anaspheric lens 140.

The light beam 120 from the EEL 100 diverges faster along one axis thanalong the other. This is true for both Fabry-Perot (FP) type EELs anddistributed feedback (DFB) type EELs. As a result, light from an EELcollimated by a simple lens produces an elliptical beam profile. In oneimplementation of the present invention, as shown in FIG. 9, a TIRsurface 180 may be shaped so that it acts as a cylindrical lens. The TIRsurface 180 can speed up the divergence of a slower axis and/or slowdown the divergence of a faster axis. In the embodiment, the asphericlens 140 may be toric, having a different focal length along each axis.The resultant beam, defined by the NA of the EEL 100, the TIR surface180 and the aspheric lens 140 along each axis, may be designed tocollimate both axes simultaneously while producing a beam of arbitraryellipticity. In this case, the NA of the TIR surface 180 is flat alongone axis and defined by a specific sag equation along the other, and theNA of the aspheric lens 140 is defined by a different sag equation alongeach axis. In other embodiments, the desired beam profile is circular ornearly circular.

An optical multiplexer based on optical subassemblies described in FIG.3a and FIG. 9 may also be constructed, enabling the use of EELs assource lasers rather than VCSELs. The design is similar to that shown inFIG. 5b. In an alternative embodiment, the angle of the output beam inthe “xy” plane defined in FIG. 9 may be controlled by rotating the TIRsurface 180 about the “z” axis to any angle desired. When a rotationwith φ=0° is implemented, the EELs emit in a direction orthogonal to thedirection of the “zig-zag” scheme.

While the foregoing description refers to particular embodiments of thepresent invention, it will be understood that the particular embodimentshave been presented for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teachings and may be made without departing fromthe spirit thereof. The accompanying claims are intended to cover suchmodifications as would fall within the true scope and spirit of thepresent invention. The presently disclosed embodiments are therefore tobe considered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims, ratherthan the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A connector to an optical fiber, comprising: aprism that includes a triangular wedge element having a first surface, asecond surface and a base; a ferrule to guide the optical fiber so as tocontact the optical fiber with the first surface of the prism, the firstsurface being substantially perpendicular to the optical fiber; and anaspheric lens integrated on the second surface, the integrated asphericlens being positioned so that the prism serves to redirect a light beamat an angle relative to an axis of an optical source input through totalinternal reflection by utilizing the base of the triangle wedge element,the aspheric lens serving to at least one of collimate the redirectedlight beam and focus the light beam before being redirected.
 2. Aconnector to an optical fiber, comprising: a prism that includes atriangular wedge element having a first surface, a second surface and abase; a ferrule to guide the optical fiber so as to contact the opticalfiber with the first surface of the prism, the first surface beingsubstantially perpendicular to the optical fiber; and an aspheric lensintegrated on the second surface, the integrated aspheric lens beingpositioned so that the prism serves to redirect a light beam at an anglerelative to an axis of an optical source input through total internalreflection by utilizing the base of the triangle wedge element, theaspheric lens serving to at least one of collimate the redirected lightbeam and focus the light beam before being redirected, wherein theconnector is a fiber collimator.
 3. A connector to an optical fiber,comprising: a prism that includes a triangular wedge element having afirst surface, a second surface and a base; a ferrule to guide theoptical fiber so as to contact the optical fiber with the first surfaceof the prism, the first surface being substantially perpendicular to theoptical fiber; and an aspheric lens integrated on the second surface,the integrated aspheric lens being positioned so that the prism servesto redirect a light beam at an angle relative to an axis of an opticalsource input through total internal reflection by utilizing the base ofthe triangle wedge element, the aspheric lens serving to at least one ofcollimate the redirected light beam and focus the light beam beforebeing redirected, wherein the connector is a fiber coupler.
 4. A fibercollimator, comprising: a prism that includes a triangular wedge elementhaving a first surface, a second surface and a base; a ferrule to guidean optical source input to the fiber collimator so as to contact theoptical source input with the first surface of the prism, the firstsurface being substantially perpendicular to the optical source input;and an aspheric lens integrated on the second surface, the integratedaspheric lens being positioned so that the prism serves to redirect alight beam at an angle relative to an axis of the optical source input,and the aspheric lens serves to collimate the redirected light beam, thebase of the triangle wedge element redirecting the light beam by totalinternal reflection (TIR).
 5. The fiber collimator of claim 4, whereinthe triangular wedge element is an isosceles triangle wedge, the lengthof the first surface being equal to the length of the second surface. 6.The fiber collimator of claim 4, wherein the prism further comprises aspacer element, the spacer element providing a mechanism to adjust anoptical path length from the aspheric lens to the optical source input,allowing the focal length of the aspheric lens, and thereby the radiusof the collimated light beam, to be adjusted while keeping thedimensions of the triangle wedge element constant.
 7. The fibercollimator of claim 4, wherein diamond-turned inserts are utilized todefine optical quality surfaces, including those for at least one of theprism, the aspheric lens and the TIR surface.
 8. A fiber coupler,comprising: a prism that includes a triangular wedge element having afirst surface, a second surface and a base; an aspheric lens integratedon the second surface, the integrated aspheric lens receiving a lightbeam, the aspheric lens being positioned so that the light beam isfocused after passing through the aspheric lens, creating a focal spotimage; and a ferrule to guide an optical fiber of the fiber coupler soas to contact an optical fiber core of the optical fiber with the firstsurface of the prism at or near the location of the focal spot image,wherein the base of the triangle wedge element serves to redirect thefocused light beam by total internal reflection (TIR) at an anglerelative to an axis of the optical fiber, the focused light beam beingdirected into the optical fiber core.
 9. The fiber coupler of claim 8,wherein the triangular wedge element is an isosceles triangle wedge, thelength of the first surface being equal to the length of the secondsurface.
 10. The fiber coupler of claim 8, wherein the prism furthercomprises a spacer element, the spacer element providing a mechanism toadjust an optical path length from the aspheric lens to the opticalfiber, allowing the focal length of the aspheric lens, and thereby thenumerical aperture of the light delivered to the optical fiber, to beadjusted while keeping the dimensions of the triangle wedge elementconstant.
 11. The fiber coupler of claim 8, wherein the light beamreceived by the aspheric lens is an elliptically shaped, collimatedlight beam and the focal spot imaged onto the fiber core is circular orsubstantially circular, the base of the triangle wedge element havingcurvature to enable this TIR surface to act as a cylindrical mirror, theaspheric lens being toric with its principle axes aligned with those ofthe cylindrically curved TIR surface, the combination of thecylindrically curved TIR surface and the toric aspheric lens serving tocollimate and correct for spherical aberrations and rendering the focalspot imaged onto the fiber core circular or substantially circular. 12.The fiber coupler of claim 8 wherein the lens parameters for theaspheric lens is optimized by utilizing a source with a numericalaperture that completely fills the full aperture of the lens.
 13. Acollimating element, comprising: a prism that includes a triangularwedge element having a first surface, a second surface and a base, thebase of the triangle wedge element having curvature to enable it to actas a cylindrical mirror to redirect the light beam by total internalreflection; and an aspheric lens integrated on the second surface, theaspheric lens being toric with principle axes aligned with those of thecylindrically curved base of the triangle wedge element, the integratedaspheric lens being positioned so that a chief ray of the light beampasses directly through the axis of the aspheric lens, wherein the lightbeam from an optical source input is an elliptically shaped beam, theelliptically shaped beam being redirected at an angle relative to anaxis of the optical source input by the cylindrically curved base, theredirected light beam being collimated by the aspheric lens, thecollimated light beam being a circularly or substantially circularlyshaped beam, wherein the aspheric lens serves to collimate theredirected light beam, the base of the triangle wedge elementredirecting the light beam by total internal reflection.
 14. Thecollimating element of claim 13, wherein the optical source input is anedge-emitting laser.
 15. A collimating optical subassembly forcollimating and redirecting a divergent light beam from a point source,comprising: an aspheric lens that receives and collimates the divergentlight beam, creating a collimated light beam; a spacer element above theaspheric lens; and a wedge element that refracts the collimated lightbeam into air at an angle relative to the axis of the aspheric lensconsistent with Snell's law, the wedge element being positioned abovethe spacer element, wherein the collimating optical subassembly isfabricated of optically transparent material and integrated as a singlepart using injection-molding techniques.
 16. The collimating opticalsubassembly of claim 15, wherein the point source is a vertical cavitysurface emitting laserdiode.
 17. The collimating optical subassembly ofclaim 15, wherein the spacer element is inserted to allow moltenoptically transparent material to more easily flow through a mold forfabricating the collimating optical subassembly using standard injectionmolding techniques.
 18. The collimating optical subassembly of claim 15,wherein the prism is made of an optically transparent material, theoptically transparent material including any one of polycarbonate,polyolefin and polyethylimide.
 19. The collimating optical subassemblyof claim 15, wherein the aperture of the aspheric lens is made largerthan the waist of collimated light beam outputted from the wedgeelement.
 20. A focusing optical subassembly for redirecting and focusinga collimated light beam, comprising: a wedge element that receives thecollimated light beam traveling in air; a spacer element below the wedgeelement; and an aspheric lens below the spacer element, wherein thefocusing optical subassembly is fabricated of optically transparentmaterial and integrated as a single part using injection-moldingtechniques, and wherein the collimated light beam received by the wedgeelement travels in air at an angle relative to an axis of the asphericlens, the wedge element redirecting a chief ray of the collimated beamthrough the spacer element along the axis of the aspheric lens, theaspheric lens focusing the collimated light beam to a point along itsaxis.
 21. A focusing optical subassembly for redirecting and focusing acollimated light beam, comprising: a wedge element that receives thecollimated light beam traveling in air; a spacer element below the wedgeelement; and an aspheric lens below the spacer element, wherein thefocusing optical subassembly is fabricated of optically transparentmaterial and integrated as a single part using injection-moldingtechniques, wherein the collimated light beam received by the wedgeelement travels in air at an angle relative to an axis of the asphericlens, the wedge element redirecting a chief ray of the collimated beamthrough the spacer element along the axis of the aspheric lens, theaspheric lens focusing the collimated light beam to a point along itsaxis, and wherein a photodetector resides at the point to which thecollimated light beam is focused by the aspheric lens.
 22. A focusingoptical subassembly for redirecting and focusing a collimated lightbeam, comprising: a wedge element that receives the collimated lightbeam traveling in air; a spacer element below the wedge element; and anaspheric lens below the spacer element, wherein the focusing opticalsubassembly is fabricated of optically transparent material andintegrated as a single part using injection-molding techniques, andwherein the collimated light beam received by the wedge element travelsin air at an angle relative to an axis of the aspheric lens, the wedgeelement redirecting a chief ray of the collimated beam through thespacer element along the axis of the aspheric lens, the aspheric lensfocusing the collimated light beam to a point along its axis, andwherein the spacer element is inserted to allow molten opticallytransparent material to more easily flow through a mold for fabricatingthe focusing optical subassembly using standard injection moldingtechniques.